Fusion welding of two metal parts involves applying a high energy heat source in a restricted region in the area where the two parts are to be welded. This results in the melting of a thin layer of metal on the surface of each part. The melted portions of each part mix together and, upon cooling, solidify and form the weld joint which fastens the two parts together.
In the welding of certain parts, the weld region must be restricted, and the penetration of the weld must be controlled. While limitations placed on the welding operation can generally control the depth of the weld effectively, many applications require a post weld inspection to verify the extent of the penetration of the weld. To avoid damage to, and subsequent loss of, the part, nondestructive inspection methods have been developed. These include, for example, ultrasonic and x-ray inspection techniques.
An example of an application where the depth of penetration of the weld must be controlled involves the repair of an ignitor boss on a gas turbine engine diffuser case. Repair is necessary if, for example, the ignitor boss has been damaged during manufacture or has incurred excessive wear in service. The construction of the diffuser case is such that the structure around the ignitor bosses is highly stressed. When the bosses are repaired by machining out the damaged area and welding in a repair insert, penetration of the weld in the boss would increase the stresses in the radius region at the bottom of the hole due to the residual stresses generated during the welding operation.
Limiting the penetration of the weld to less than the depth of the cavity machined out for the insert would permit a localized precipitation heat treatment of the weld, whereas a weld which had penetrated the full depth of the machined out cavity would require a full solution heat treatment of the entire diffuser case followed by precipitation heat treatment to restore the required mechanical properties. The full solution heat treatment would add considerable expense due to furnace operating requirements, removal of accessory hardware from the case, and removal of protective coatings, and would incur the risk of distortion of the diffuser case.
While it is desirable to limit the depth of penetration of the weld in this application, it is also necessary to insure a minimum depth of penetration in order to properly secure the repair insert in position. A nondestructive inspection method, which would verify adequate, but not excessive, weld penetration, is needed to insure the integrity of the repair operations.
Murakami, in Japanese Patent No. 58-179577, teaches that a minimum depth of penetration of a weld can be assured by detecting light or radioactive rays emitting from the weld zone and passing to the outside of the welded structure through a passage provided through the weld structure at a point near the deepest point of the required weld penetration. The positioning of a second passage and detection system is also disclosed to assure that allowable penetration has not been exceeded. This method requires either the existence or the creation of suitable pathways for the radiating signals to reach the detection system.
Leroux, in U.S. Pat. No. 4,893,944, teaches a nondestructive method of evaluating a joint made by hot pressure welding two metal parts at an elevated forging temperature. A tensile stress is applied perpendicularly to either side of the joint surface while the joint region is relatively hot, in order to enlarge any defects present in the joint surface. Such enlarged defects (if they exist) can be detected by means of x-rays, gamma rays, ultra-sound or a like system. This system would apparently be applicable only where some degree of deformation of the welded system is allowable, and in which the weld interface is planar, so that a uniform tensile force can be applied normal to the weld interface.
Sayer, et al, in U.S. Pat. No. 3,436,515, disclose electron beam welding of workpieces arranged in a butting relationship wherein each workpiece is shaped with matching grooves which provide one or more recesses along the joint line so that the work pieces can be butt welded together by beams from opposite sides which penetrate only as far as the recesses. This prevents or reduces the risk of the weld formed by a beam on one side impinging on the weld formed by the beam from the other side. The grooves which limit the penetration of the weld beam appear to remain as grooves in the finished product.